Tracing the migration of mantle CO2 in gas fields and mineral water springs in south-east Australia using noble gas and stable isotopes

Karolyte, R., Johnson, G., Gyore, D. , Serno, S., Flude, S., Stuart, F. M. , Chivas, A. R., Boyce, A. and Gilfillan, S. M.V. (2019) Tracing the migration of mantle CO2 in gas fields and mineral water springs in south-east Australia using noble gas and stable isotopes. Geochimica et Cosmochimica Acta, 159, pp. 109-128. (doi: 10.1016/j.gca.2019.06.002)

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Geochemical monitoring of CO2 storage requires understanding of both innate and introduced fluids in the crust as well as the subsurface processes that can change the geochemical fingerprint of CO2 during injection, storage and any subsequent migration. Here, we analyse a natural analogue of CO2 storage, migration and leakage to the atmosphere, using noble gas and stable isotopes to constrain the effect of these processes on the geochemical fingerprint of the CO2. We present the most comprehensive evidence to date for mantle-sourced CO2 in south-east Australia, including well gas and CO2-rich mineral spring samples from the Otway Basin and Central Victorian Highlands (CVH). 3He/4He ratios in well gases and CO2 springs range from 1.21 to 3.07 RA and 1.23 – 3.65 RC/RA, respectively. We present chemical fractionation models to explain the observed range of 3He/4He ratios, He, Ne, Ar, Kr, Xe concentrations and δ13C(CO2) values in the springs and the well gases. The variability of 3He/4He in the well gases is controlled by the gas residence time in the reservoir and associated radiogenic 4He accumulation. 3He/4He in CO2 springs decrease 29 away from the main mantle fluid supply conduit. We identify one main pathway for CO2 supply to the surface in the CVH, located near a major fault zone. Solubility fractionation during phase separation is proposed to explain the range in noble gas concentrations and δ13C(CO2) values measured in the mineral spring samples. This process is also responsible for low 3He concentrations and associated high CO2/3He, which are commonly interpreted as evidence for mixing with crustal CO2. The elevated CO2/3He can be explained solely by solubility fractionation without the need to invoke other CO2 sources. The noble gases in the springs and well gases can be traced back to a single end-member which has suffered varying degrees of radiogenic helium accumulation and late stage degassing. This work shows that combined stable and noble gas isotopes in natural gases provide a robust tool for identifying the migration of injected CO2 to the shallow subsurface.

Item Type:Articles
Additional Information:This work was supported by an EPSRC PhD studentship in partnership with CO2CRC and Badley Geoscience Ltd. G. Johnson and S. Gilfillan were partially supported by both UKCCSRC and Scottish Carbon Capture and Storage (SCCS), S. Serno was funded by the UK Carbon Capture and Storage Research Centre (UKCCSRC) Call 2 grant. S. Flude was supported by by EPSRC grant EP/K036033/1.
Glasgow Author(s) Enlighten ID:Boyce, Professor Adrian and Gyore, Dr Domokos and Stuart, Professor Fin
Authors: Karolyte, R., Johnson, G., Gyore, D., Serno, S., Flude, S., Stuart, F. M., Chivas, A. R., Boyce, A., and Gilfillan, S. M.V.
Subjects:Q Science > QE Geology
College/School:College of Science and Engineering > School of Geographical and Earth Sciences
Journal Name:Geochimica et Cosmochimica Acta
ISSN (Online):0016-7037
Published Online:11 June 2019
Copyright Holders:Copyright © 2019 Elsevier
First Published:First published in Geochimica et Cosmochimica Acta 259:109-128
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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